Energy fluxes and cascades in models of ocean circulation

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Understanding ocean circulation involves understanding how mechanical energy is transferred from the basin scale where it is added by the winds to the centimetre scale where it is converted to heat. Ocean currents and eddies are in approximately geostrophic balance (Coriolis and horizontal pressure gradient forces) on length scales down to 10km. How energy is transferred from the basin scale to the ocean mesoscale (10-50km) is reasonably well understood, although part of this proposal corrects some misconceptions relating to this process. A bigger problem is getting energy from the mesoscale to the smaller submesoscale. The mesoscale is characterized by geostrophic turbulence which, unlike 3d turbulence, transfers energy in the inverse sense: from small to large scales. Dissipation then requires a transfer of energy from the nearly-geostrophic circulation to ageostrophic modes such as inertial oscillations. Much work suggests these transfers to be weak in the interior of the water column. Observations, however, show a rich field of near-inertial motion (frequencies close to the Coriolis frequency) super-imposed on the geostrophic flow. These are forced in large part by high frequency events (stroms, tides, etc) that are often ignored in theoretical studies. In other words, the background level of near-inertial motion in the ocean is considerably higher than it is in most studies aimed at quantifying these sorts of energy transfers. High frequency forcing increases the background level of the near-inertial energy. Our basic thesis is that this can significantly enhance the geostrophic to ageostrophic energy transfer mechanism and - even where it does not - will affect the vertical velocity field (and hence transport of tracers). We consider various forcing mechanisms to excite the unbalanced motion; for example, a sea surface temperature dependence in the wind stress divergence which may add significant ageostrophic motion to strong ocean currents (where geostrophic to ageostrophic transfers more readily occur). Finally, because much of the wind energy input occurs over the Southern Ocean, some of our work will relate to an understanding of the Antarctic Circumpolar Current.